Observing
The duration of this shower extends from April 18 to April 25. The short-duration maximum occurs during April 23-24, from a radiant of RA=112 deg, DECL=-43 deg. The shower is associated with the periodic comet Grigg-Skjellerup. Although this comet was officially discovered in 1902, it was only recently perturbed by Jupiter into a close-approach orbit with Earth. Activity was first noted in 1972, and visual hourly rates of 18 to 42 meteors per hour were noted during the comet’s perihelion returns of 1977 and 1982. Activity levels are typically very low or nonexistent in other years.
History
During 1971, H. B. Ridley was examining the predicted orbital elements for the coming return of periodic comet Grigg-Skjellerup, when he noticed that Earth would make a close approach to the comet’s orbit on April 23.02, 1972. The separation between the orbits was calculated as only 0.004 AU, while the encounter was to occur only 50 days after the passage of the comet. The predicted radiant was RA=107.5 deg, DECL=-45 deg.
Observations made during the predicted appearance of this meteor shower revealed a very poor display. During the period of April 16-23, 17 observers in the United States obtained average hourly rates of only 1.9, with a maximum of about 4 per hour being observed by B. Edwards (Jacksonville, Florida) during a three hour interval on April 18/19. Observers in West Australia were met with even weaker activity, as 7 observers compiled 70 hours of searching during April 21-24, only to detect three possible shower members during an eight-hour interval on the night of April 22/23. A more positive Southern Hemisphere observation was made by W. J. Baggaley (University of Canterbury, New Zealand), who utilized radio equipment in his search. He detected an “increase in the rate of radio-meteor echoes over the normal sporadic activity on the four days 1972 April 21, 22, 23 and 24….” He added that the observed activity flux was consistent with a radiant at RA=107.5 deg, DECL=-45 deg, though the rates were considered too low for an accurate radiant determination.
Comet Grigg-Skjellerup was next expected at perihelion in 1977. Predictions for a possible meteor shower in this year had actually been first made by G. Sitarski during 1964. He said that on April 23.3, 1977, Earth would cross the comet’s orbit just 12 days after the comet, and that activity would probably emanate from RA=109.6 deg, DECL=-44.3 deg. Once again, observers in the United States were not successful in observing activity, but circumstances were different in West Australia.
Jeff Wood, A. Saare and G. Blencowe, observing in Perth, West Australia, individually observed maximum rates of 18-24 meteors per hour during a three-hour interval centered on April 23.5, 1977. Numerous meteors were plotted, which revealed a radiant of RA=112 deg, DECL=-43 deg, and the ZHR was calculated as 36.47±2.61. The overall duration of the activity was given as April 23-25. The meteors were typically bright and slow.
A weak return of the Pi Puppids was observed by the West Australian meteor observers during 1979—a time when the comet was actually nearing its greatest distance from the sun. Activity was noted from April 21 to 24, with a maximum ZHR of 3.54±1.77 coming on April 23. The average radiant was given as RA=112 deg, DECL=-43 deg.
The comet’s next perihelion passage came on May 14, 1982. A very strong return of the Pi Puppids was observed on the night of April 23/24. The first detection of increased activity was made by A. Gozalos Beltran (Cochabamba, Bolivia) when 58 meteors were detected during a period of 1 hour 35 minutes. He described the meteors as being predominantly yellow. A strong return was also noted in West Australia. Individuals reported 25-42 meteors per hour, with the ZHR reaching 22.8 on April 24.49. By April 24.56, the ZHR had dropped to 7.1. The West Australian observers reported that 56.5% of the meteors were yellow, while 19.6% were orange. Trains were observed among 16.1% of the meteors and the average magnitude of 447 meteors was 1.97. It is interesting that one year later, West Australian observers detected a maximum ZHR as high as 12.7 on April 23/24, and estimated the average meteor magnitude as 2.33.
This meteor stream is definitely associated with periodic comet Grigg-Skjellerup and is very new, as is evidenced by the near total lack of activity in years when the comet is not at perihelion. Therefore the difference between the orbits of the comet and meteor stream is negligible.